The present invention relates to threaded column clamp assemblies used to secure a member with respect to a column and, more particularly, to a threaded clamp assembly having a differential pitch thread to distribute load forces on the threads.
In many types of presses, it is necessary to secure a movable crosshead of a press at one or more positions relative to a stationary member or base of the press. For example, injection molding machines commonly comprise a two-part mold, wherein one half of this mold is mounted on a lower platen while the other half is mounted on an upper movable platen or crosshead which allows the mold to be opened and closed. The mold is typically opened and closed with one or more driving ram or cylinder assemblies. Moldings are produced by injecting the material to be molded into the mold cavity under pressure. In the course of such injection, a force is built up which tends to separate the lower platen from the upper platen to open the mold. In order to prevent mold opening, the crosshead must be held stationary with respect to the base of the press with a force at least as strong as the force created by the injected material. However, rapid opening and closing of the mold is also desired in order to achieve economically advantageous production quantities.
Cylindrical threaded column clamps have proven to be an effective clamping apparatus to overcome such molding forces. One such column clamp assembly is disclosed in U.S. Pat. No. 4,509,910 issued to Petersen. The clamp assembly includes an inner clamp sleeve positioned concentrically about each column. The inner clamp sleeve is slidable with respect to a longitudinal column axis and has an upper unthreaded portion and a lower threaded portion. On a side opposite the threaded and unthreaded portions is an inner generally cylindrical surface for engaging the column. An outer threaded clamp sleeve is secured to the crosshead assembly and concentrically around the inner clamp sleeve. The inner and outer clamp sleeves are threaded together to limit longitudinal movement of one clamp sleeve with respect to the other. Although threaded together, there is sufficient clearance between the opposing threads of the inner and outer clamp sleeves to define a helical chamber. A hydraulic fluid system is provided for selectively introducing pressurized hydraulic fluid into the helical chamber. The pressurized hydraulic fluid causes the inner clamp sleeve to contract radially so that its inner cylindrical surface is urged substantially and uniformly against the column to limit longitudinal movement of the clamp assembly with respect to the column. When subjected to perpendicular loading forces such as injection mold forces, the interlocking threads of the inner and outer sleeves prevent longitudinal movement of the clamp assembly.
A second form of a column clamp assembly is disclosed in U.S. Pat. No. 4,907,959 issued to Hauch. The Hauch patent teaches a clamp assembly similar to that illustrated in the Petersen patent; however, the Hauch patent secures the clamp assembly to a base portion of the press while a plurality of columns are secured to a moveable crosshead wherein each column is positioned within the corresponding clamp assembly. The crosshead and attached columns are vertically adjusted with lift actuators positioned between a fixed portion of the press and the crosshead. When a desired crosshead position has been reached, hydraulic pressurized fluid operates each clamp assembly, in the manner taught by the Petersen patent and described above, to secure the crosshead position.
Although the clamp assemblies described above are effective clamping devices for columns of a press, the clamp assemblies appear to suffer from stress fatigue that can result in a relatively short useful life. Research by this inventor indicates that this stress is attributed to pressurization of the helical chamber between the inner and outer clamping sleeves wherein a slight difference in thread pitch between the inner and outer sleeves is caused by Poisson strain. Specifically, this research indicates that the thread pitch on the inner clamping sleeve increases slightly due to inward radial displacement of the inner sleeve against the column, and that the thread pitch on the outer sleeve decreases slightly due to outward radial displacement of that sleeve. With interlocking thread contact on a first or upper end of the clamp assembly when the clamp assembly is stationary and under load, such as mold injection loading, a relative pitch difference then accumulates over the longitudinal length of the clamp to produce a thread gap at a second or lower end of the clamp. Since the second end threads are not in contact with each other, a substantial amount of the load is applied to the first end threads which causes stress fatigue thereof that in turn reduces the life of the clamp.
In addition to higher stress loads on the first end threads due to Poisson strain of the inner and outer sleeves from the clamping pressures described above, increased force loads are further exerted on the first end threads. This additional force load, herein referred as "end-effect loading", is attributed to the unthreaded portion of the inner sleeve wherein frictional load forces between the inner surface of the unthreaded portion and the outer surface of the column are transferred to the first end threads. In other words, the first end threads are subjected to frictional load forces associated with the unthreaded portion of the inner sleeve as well as the frictional forces associated with its corresponding threaded portion. As with the unequal loading associated with the Poisson strain described above, this end-effect loading also contributes to stress fatigue of the first end threads which reduces the life of the clamp.